When a fluid is subjected to a temperature change, its material properties, such as density and viscosity, change accordingly. In some situations, these changes are large enough to have a substantial influence on the flow field. Because the fluid transports heat, the temperature field is in turn affected by changes in the flow field. Hence, a nonisothermal flow is always a two-way coupling between fluid flow and heat transfer.

This phenomenon often shows up in heat exchangers, chemical reactors, or in processes where components are cooled. Studies of temperature-dependent flow are also of interest to those analyzing weather patterns and oceanographic flows.

Free Convection in a Cold Water Glass

This example considers the natural convection of a glass of chilled water that sits on a fixed-temperature surface in an environment at the same temperature. The animation shows the fluid’s velocity in the glass as it heats up over time.